As one such fluid machine, a vacuum pump disclosed in Patent Document 1, for example, has been proposed. The vacuum pump of Patent Document 1 includes a housing formed by a rotor housing member, a front housing member, and a rear housing member. The front housing member is joined to a front end of the rotor housing member. The rear housing member is joined to a rear end of the rotor housing member. The rotor housing member is a cylinder block configured by a pair of upper and lower block pieces. A pair of rotary shafts are rotatably supported by the front housing member and the rear housing member each through a radial bearing. A plurality of rotors are fixed to each of the rotary shafts. The rotary shafts rotate synchronously through engagement between gears each secured to an end of the corresponding rotary shaft. Each of the radial bearings is supported by a bearing holder, and the bearing holder is fixedly fitted in an engagement hole formed in an end surface of the rear housing member.
The housing of the vacuum pump is assembled in the following manner. The rotary shafts are supported by a lower one of the block pieces, or a lower block piece. Then, an upper block piece is joined to the lower block piece to form the cylinder block. The front housing member and the rear housing member are then joined to the cylinder block. Subsequently, the bearing holders, to which the radial bearings are attached, are fitted in the engagement holes of the rear housing member along the axial directions of the rotary shafts, which are supported by the housing. The vacuum pump is thus completed. Specifically, prior to joining the upper block piece with the lower block piece, the clearances between the rotors and the inner surface of the cylinder block facing the rotors are adjusted. Before joining the upper block piece to the lower block piece, the engagement positions of the gears, which are secured to the ends of the respective rotary shafts, are adjusted so as to provide a proper phase difference between each engageable pair of the rotors of the two rotary shafts.
In the vacuum pump of Patent Document 1, if the clearances between the rotors and the inner surface of the cylinder block or the phase difference between each engageable pair of the rotors is not appropriate after the housing has been assembled, it is necessary to repeat the adjustment of the clearances or the phase difference. Such readjustment is performed as follows. The radial bearings and the bearing holders are removed from the rear housing member, and the front housing member and the rear housing member are separated from the cylinder block. The upper block piece is then removed from the lower block piece. As a result, the vacuum pump of Patent Document 1 requires complicated assembly of the housing and complicated readjustment after completion of the assembly of the housing.
Patent Document 2 proposes a fluid machine that simplifies assembly of a housing. The fluid machine of Patent Document 2 is a multistage vacuum pump having a casing (a housing) with a two-piece structure that can be divided into upper and lower pieces. The casing includes a plurality of pump operation chambers. The fluid machine is assembled simply by joining an upper casing member with a lower casing member after supporting a pair of rotary shafts, to which a plurality of rotors are fixed, by means of the lower casing member each through a bearing and a shaft sealing device. In the fluid machine of Patent Document 2, before the upper casing member is joined to the lower casing member, the clearances between the rotors and the inner surfaces of the pump operation chambers are adjusted. Further, engagement positions of timing gears, which are each secured to an end of the corresponding rotary shaft, are adjusted so as to ensure an appropriate phase difference between each engageable pair of the rotors between the two rotary shafts.
However, in assembly of the casing of the fluid machine of Patent Document 2, when the rotary shafts are supported by the lower casing member through the respective bearings, the bearings separate from the lower casing member. If the phase difference between each engageable pair of the rotors is adjusted with the bearings separated from the lower casing member, the phase difference cannot be set to an appropriate value. Further, if the upper casing member is joined to the lower casing member in this state, the fluid machine is assembled with the phase difference maintained as an inappropriate value.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-257244    Patent Document 2: Japanese Laid-Open Patent Publication No. 4-132895